05000499/LER-2014-001
| South Texas Unit 2 | |
| Event date: | 12-31-2013 |
|---|---|
| Report date: | 03-17-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 4992014001R00 - NRC Website | |
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I. DESCRIPTION OF EVENT
A. REPORTABLE EVENT CLASSIFICATION
This event is reportable pursuant to 10 CFR 50.73(a)(2)(i)(B), any operation or condition which was prohibited by the plant's Technical Specifications. The event is also reportable pursuant to 10 CFR 50.73(a)(2)(v)(D), any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident.
B. PLANT OPERATING CONDITIONS PRIOR TO EVENT
Unit 2 was operating in Mode 1 at 99.5% power.
C. STATUS OF STRUCTURES, SYSTEMS, AND COMPONENTS THAT WERE INOPERABLE AT THE
START OF THE EVENT AND THAT CONTRIBUTED TO THE EVENT
The event resulted from the inoperability of Standby Diesel Generator (SDG) 23 due to leakage from an aluminum-bronze nipple and tee assembly. There were no other structures, systems, or components that were inoperable at the start of the event that contributed to the event.
D. NARRATIVE SUMMARY OF THE EVENT
On October 3, 2013, an approximately one gpm leak was identified on SDG 22 due to unsatisfactory thread engagement and only slight erosion was noted. This condition was immediately documented and repaired. Work orders were then scheduled to replace the aluminum-bronze tees and piping nipples on SDG 23 and SDG 11 with stainless steel components using an existing design change package (DCP).
The aluminum-bronze components had already been replaced with stainless steel components for SDGs 13,21 and 12.
On November 6, 2013, during a monthly surveillance run of SDG 23, a 60 drop per minute (dpm) leak was identified from a half-inch aluminum-bronze threaded tee connecting the intercooler continuous flowing vents to the Essential Cooling Water (ECW) return piping. This condition was evaluated by Operations and the SDG was declared Operable but Degraded.
During a monthly surveillance run of SDG 23 on December 31, 2013 at 13:46 hours, an approximately three gpm ECW leak was discovered on SDG 23 originating from the same location as the smaller leak identified on November 6, 2013. Operations subsequently declared SDG 23 inoperable and non- functional. The leaking nipple and tee assembly was replaced with stainless steel components on December 31, 2013. SDG 23 was declared operable January 1, 2014.
A Reportability Review completed on January 16, 2014 determined this condition was reportable as a condition prohibited by Technical Specifications pursuant to CFR 50.73(a)(2)(i)(B) and as a condition that could have prevented fulfillment of a safety function reportable pursuant to CFR 50.73(a)(2)(v).
E. METHOD OF DISCOVERY
Both the initial 60 dpm leak on November 6, 2013 and the three gpm leak on December 31, 2013 were discovered during monthly surveillance runs of SDG 23.
Following a reportability review completed on January 16, 2014, the event was determined to be reportable as a condition prohibited by Technical Specifications and a condition that could have prevented the fulfillment of a safety function.
II. EVENT-DRIVEN INFORMATION
A. SAFETY SYSTEMS THAT RESPONDED
The SDG leaks were discovered during surveillance activities. No safety systems were required to respond to this event.
B. DURATION OF SAFETY SYSTEM INOPERABILITY
SDG 23 was determined to be inoperable from the time of discovery of the initial leak on November 6, 2013 at approximately 1330 hours0.0154 days <br />0.369 hours <br />0.0022 weeks <br />5.06065e-4 months <br /> until the time SDG was declared operable following maintenance and post-maintenance testing on January 1, 2014 at 0157 hours0.00182 days <br />0.0436 hours <br />2.595899e-4 weeks <br />5.97385e-5 months <br />, a time period of approximately 55 days 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> and 27 minutes, which exceeded the 14-day allowed outage time allowed by Technical Specification 3.8.1.1. During the time period when SDG 23 was inoperable, there were short periods when one of the other SDGs was also inoperable. Therefore, in addition to being a condition prohibited by Technical Specifications this event is also reportable as a condition that could have prevented the fulfillment of a safety function.
C. SAFETY CONSEQUENCES AND IMPLICATIONS OF THE EVENT
The event did not have an adverse effect on the health and safety of the public.
A risk assessment was performed for this event. The assessment concluded that, given a small leak of three gpm, SDG 23 would fail in 1.5 days due to flooding of the room; however, SDG 23 would still meet the PRA mission time of 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. Credit was given for the performance of operator rounds which would give sufficient time to discover the leak and to implement mitigating strategies. In the case of a Loss of Offsite Power (LOOP), the assessment concludes that the LOOP non-recovery probability is low and recovery is likely to occur before the SDG fails. The calculated incremental core damage probability and incremental large early release probability are very small for this event.
III. CAUSE OF THE EVENT
The cause of the event is erosion of the aluminum-bronze nipple and tee assembly that led to a through-wall ECW leak.
IV. CORRECTIVE ACTIONS
The aluminum-bronze tee and piping nipple for SDG 23 were replaced with stainless steel components on December 31, 2013 using a previously approved design change. On January 8, 2014 the aluminum-bronze tee and piping nipple for SDG 11 were also replaced, completing the replacement of the aluminum-bronze tee assemblies with stainless steel components for the six SDGs at STPEGS.
V. PREVIOUS SIMILAR EVENTS
There have been no similar reportable events at STP related to erosion based degradation within the last three years that have occurred for the same reason as this event.
VI. ADDITIONAL INFORMATION
None